The present invention relates to a vacuum pump, for example, for use in semiconductor manufacturing process.
As a vacuum pump for use in semiconductor manufacturing equipment, a screw vacuum pump is disclosed in Japanese Patent Application Publication No. 5-164076 (cf. Pages 3-5 and FIG. 1). The screw vacuum pump has a pair of screw rotors (20 and 21) disposed in a rotor housing (11). The rotors (20 and 21) are mounted on rotor shafts (24 and 25) which are supported by a gear casing (12), respectively. When the rotor shafts (24 and 25) are rotated, the vacuum pump sucks thereinto air from outside of the screw vacuum pump, compresses it and discharges the compressed air to outside of the screw vacuum pump. The rotor shaft (24) is supported by two roller bearings (30 and 31). The bearing (30) is disposed in the cylindrical portion (33) of the gear casing (12), and the bearing (31) is supported by the lower receiving portion of the gear casing (12). Similarly, the rotor shaft (25) of the rotor (21) is supported by the gear casing (12).
The oil pan of a lower casing (12B) stores therein a lubricating oil in which the lower ends of the rotor shafts (24 and 25) are immersed. The rotor shafts (24 and 25) have therein oil-feed holes (51 and 52) extending axially from the lower end to the middle of the rotor shafts (24 and 25). The oil-feed holes (51 and 52) have outlet ports located at a position corresponding to the bearing (30) for the rotor shaft (24) and the bearing for the rotor shaft (25), respectively. Oil-return holes (53) are formed in the cylindrical portion (33) adjacent to the outer peripheries of the bearings which support the rotary shafts (24 and 25). The oil-return holes (53) lead to the oil pan.
In the above vacuum pump, as the rotor shafts (24 and 25) are rotated, the lubricating oil stored in the oil pan is introduced through the oil-feed holes (51 and 52) of the rotor shafts (24 and 25) to the bearing (30) for the rotor shaft (24) and the bearing for the rotor shaft (25). In view of the structure shown in the drawings, although not expressly mentioned in the above-cited publication, the lubricating oil may pass as follows. Of the lubricating oil fed to the bearings, a part of the lubricating oil enters the bearings and the rest of the lubricating oil flows to the oil-return holes (53). The lubricating oil in the bearings returns to the oil pan through the clearance between the rotor shafts (24 and 25) and the gear casing (12) after lubricating and cooling the bearings. The lubricating oil flowing to the oil-return holes (53) returns to the oil pan through the oil-return holes (53).
According to the prior art vacuum pump disclosed in the above cited publication, while a part of the lubricating oil fed to the bearing (30) through the oil-feed holes (51 and 52) flows to the oil-return hole 53, the remaining substantial amount of the lubricating oil does not flow to the oil-return hole 53 but tends to remain on the bearing (30). The lubricating oil remaining on the bearing (30) directly enters the bearing (30) through the upper opening of the bearing (30) adjacent to the outlet ports of the oil-feed holes (51 and 52). The lubricating oil also tends to remain inside the bearing (30). Thus, the lubricating oil is agitated in the bearing (30) thereby to generate shearing heat in the bearing (30). This results in an increase in the drive torque and temperature of the rotary shafts (24 and 25). It is noted that the numerals in parentheses above correspond to the reference numerals used in the above-cited publication.
The present invention which has been made in view of the above-described problems is directed to a vacuum pump which appropriately feeds a lubricating oil to a bearing thereby to improve lubrication and cooling of the bearing and a rotary shaft.